Liquid crystal displays have been used in various embodiments such as watches and electronic calculators because of their thinness, lightness, and low power driving properties. With the advancement of integrated circuits (IC), liquid crystal displays have been increasing the display size and extending their use in computers, liquid crystal TV sets, etc. in place of conventional cathode-ray tubes.
However, nematic liquid crystals which have conventionally been used have a slow response as needing a switching time of from 10 to 50 milliseconds and also undergo a reduction in display contrast with increases in number of pixels and in display area.
In the state-of-the-art liquid crystal displays, the above-described disadvantages are coped with by providing a thin film transistor (TFT) on each pixel to achieve so-called active matrix driving or by increasing the angle of twist of liquid crystal molecules sandwiched between a pair of substrates to 220.degree. to 270.degree. (called super-twisted nematic mode: STN).
Mounting of TFT according to the former means not only entails very high cost but has a poor yield, resulting in an increased production cost. Cost reduction by introducing a large-scaled production line having been studied, there is a limit due to essential involvement of many production steps. Further, ever since the appearance of high-definition televisions (HDTV), there has been an increasing demand of liquid crystal displays making a high-density display. In nature of TFT and nematic liquid crystals, it is nevertheless considered very difficult to increase display density.
Although the STN mode exhibits an increased contrast ratio, it has a longer response time as long as 100 to 200 milliseconds and is thus limited in its application.
On the other hand, ferroelectric liquid crystals which were proposed by N. A. Clark, et al. as surface-stabilized ferroelectric liquid crystal devices (SSFLD) (refer to N.A. Clark, et al., Appl. Phys. Lett., Vol. 36, p. 899 (1980) have been attracting attention for their fast response reaching about a thousand times that of nematic liquid crystals.
However, such a fast response time is a result obtained in a high temperature range, and the response time obtained around room temperature is impractically as long as several tens of microseconds. Besides, there remains an unsolved problem in molecular orientation. Therefore, ferroelectric liquid crystal display elements have not yet been put to practical use. In particular, the molecular orientation of ferroelectric liquid crystals proved more complicated than thought by Clark, et al. That is, the director of liquid crystal molecules is apt to be twisted in a layer, with which a high contrast ratio cannot be achieved. Further, the layers have been believed to be aligned upright and perpendicular to the upper and lower substrates (bookshelf structure) but, in fact, were found to have a bent structure (chevron structure). As a result, zigzag defects develop to reduce a contrast ratio.
It is known that a response time of ferroelectric liquid crystals is dependent on spontaneous polarization, which develops depending on the dipole moment in the direction perpendicular to the long axis of the molecule, chirality and the orientation of the dipole, rotational viscosity, and intensity of the applied electric field. However, there is a limit of voltage which can be practically derived from an IC used in combination, and a compound having a low viscosity and exhibiting high spontaneous polarization has not been discovered. From these and other reasons, the response time of ferroelectric liquid crystals has not yet been satisfactorily improved.
In general, ferroelectric liquid crystal materials are prepared by adding an optically active compound called a chiral dopant to an achiral base liquid crystal composition showing a smectic C phase (S.sub.c phase). In many cases, phenylpyrimidine type liquid crystal compounds having advantageous viscosity properties are utilized as an achiral base. In actual use, however, properties of the resulting ferroelectric liquid crystal composition, such as viscosity and response time, greatly vary depending on the properties of optically active compounds added thereto.
Further, in order to obtain satisfactory orientation, ferroelectric liquid crystal compositions are demanded to have a smectic A phase (S.sub.A phase) and desirably a nematic phase in which orientation can be effected with relative ease in a higher temperature range.
As containing an optically active compound as mentioned above, a ferroelectric liquid crystal composition in its nematic phase shows a chiral nematic phase (N.sup.* phase) in which a helical structure is induced. If the helical pitch in the N.sup.* phase has temperature dependence, orientation would be difficult. Accordingly, the optically active compound to be added is required not only to provide a liquid crystal composition having the properties demanded as a ferroelectric liquid crystal but also to induce a chiral nematic phase whose helical pitch is less dependent on temperature.
An optically active compound is also used as a chiral dopant for nematic liquid crystal materials for use in nematic liquid crystal displays. In this case, an optically active compound is needed for preventing occurrence of so-called reverse domains in which liquid crystal molecules are twisted to an opposite direction and also for stably maintaining the angle of twist of molecules in the cell. Of the properties demanded for a chiral dopant for a nematic phase, a helical twisting power (HTP=1/concentration by weight.times.helical pitch) is the most important. A chiral dopant with a larger HTP value will be effective in a smaller amount, thus minimizing impairment of the characteristics inherent to host nematic liquid crystal mixtures.
Further, reduction of temperature dependence of a helical pitch in N.sup.* phase is an important factor for a chiral dopant to be added to nematic liquid crystals used in twisted nematic (TN) mode and super twisted nematic (STN) mode display elements. For example, if a chiral dopant shows high positive dependence on temperature (i.e., the pitch is broadened with an increase in temperature), it must be used in combination with a chiral dopant having an opposite tendency to offset the temperature dependence, which makes the chiral dopant mixing system complicated.
The chiral dopant currently used for nematic liquid crystals comprises a mixture of several kinds of optically active compounds for the purpose of controlling a helical pitch in the N.sup.* phase and reducing the temperature dependence of the helical pitch. Not a few of the known optically active compounds exhibit no liquid crystal properties and, when added to a nematic liquid crystal, cause a drop of the nematic-isotropic phase transition temperature.